In general, a weld is a localized coalescence, i.e., unified body, of materials produced by heating the materials to a suitable temperature, usually to form a joint between two or more workpieces. In metal welding, the heat is typically generated using an electrode that carries current from a current source to the workpiece(s) to be welded by placing the electrode sufficiently near the region(s) of the workpiece(s) to be welded. Conventionally, the metal of the workpiece(s) is known as the base metal. In some welding processes the electrode is a consumable electrode that provides a filler metal that adds to the amount of metal in the weld. In other welding processes the electrode is non-consumable. Non-consumable electrodes may be used either with or without a filler metal. If a filler metal is required, it may be supplied to the welding process, e.g., as a metal wire, rod, or other form.
In general, it would be highly desirable to be able to weld any type of base metal in an ambient-air environment without any detrimental effects to the weld due to the presence of the air. However, when metals are heated to welding temperatures, they can become contaminated by one or more of the components of the air, e.g., oxygen, hydrogen, and nitrogen, to form various defects within the weld and/or the region(s) adjacent the weld that are heated to a temperature high enough for such contamination to occur. These adjacent regions are typically collectively referred to as the heat-affected zone (HAZ). Different base metals have different sensitivities to air at welding temperatures. The more sensitive a base metal is to air, the more detrimental the contamination will be to the weld and/or HAZ. The presence of contaminants within a weld and/or HAZ can lead to partial or total failure of the weld.
Over the years, the welding of some metals, such as steel and aluminum, in an ambient-air environment has been improved through the use of various fluxes that vaporize during welding and provide a relatively inert, shield-like environment at the weld and HAZ during the most critical time, i.e., during the coalescing and cooling of the base metal, and the filler metal, if present. In some welding processes, these fluxes are typically provided as a coating on each consumable electrode or in a hollow core within each consumable electrode. In other welding processes, particularly submerged arc welding (SAW), fluxes are typically provided in a layer of granules that cover the region(s) to be welded. Welding is then performed through the layer of granules. Obviously, SAW must be performed where welds will be formed in a substantially horizontal plane. Properly-formed steel and aluminum welds made in an ambient air environment using these fluxes are largely free of air contamination and corresponding defects.
However, other metals, such as titanium, molybdenum, tantalum and other refractory metals, are particularly sensitive to air at welding temperatures. For example, the extreme reactivity of titanium with oxygen leads to the formation of a thin protective oxide at room temperature. However, at welding temperatures, the oxide grows and offers little or no protection to a weld and HAZ. Hydrogen contamination manifests itself as a loss of ductility, perhaps through the formation of hydride phases or as cracking due to precipitation of hydrogen within voids or cracks in the weld and/or HAZ. Thus, to produce a weld of acceptable quality in titanium, the weld and HAZ must be well shielded from contamination by air.
Most conventional techniques for welding titanium require that the regions to be welded be both free of surface contaminants and protected from exposure to air. Prior to welding, the titanium must be rigorously and completely cleaned to remove contaminants that contain carbon, oxygen, and hydrogen that can significantly compromise the quality of the weld. For example, the oxide scale may be completely removed using a method such as acid pickling with a solution containing hydrofluoric and nitric acids.
The cleaned titanium must then be protected from air and its constituent components. In general, there are two common conventional techniques for protecting titanium during welding. These are: (1) welding inside a chamber filled with one or more inert gases; and (2) providing a trailing shield large enough to protect the weld and HAZ. These two techniques are generally associated with parts of different sizes. That is, small parts are typically welded in an enclosed chamber. Cleaned tools and parts are loaded into the chamber, followed by backfilling of the chamber with one or more inert gases, such as argon or helium. Welding takes place after all air is purged from the chamber.
In contrast, large parts are typically welded by providing a shroud for the weld region and entire HAZ for containing one or more inert gases, such as argon or helium. Shrouds can be custom made for each weld geometry. The welding tool, which typically includes an electrode surrounded by an annular passageway for providing shielding gas, must not itself entrain air. Trailing shields are used to protect the metal, both weld and HAZ, behind the tool as the tool is moved along the weld joint. The back side of the welded joint must also be protected with shielding gas from a back-side shroud system. A back-side shrouding fixture must fit with virtually no gap between it and the titanium parts since air may be drawn into the shroud system, resulting in contamination.
Conventional techniques for welding titanium are generally very cumbersome and labor intensive and are a major impediment to the widespread use of titanium and other air-sensitive metals in situations where welding is desirable or required. A less costly and more convenient technique for protecting titanium from reactive ambient-air environments during welding of the titanium could lead to more widespread use of titanium and other air-sensitive metals.